郑和第一次出使西洋的时间是(   )

活动一：探究白色沉淀溶解的生成什么？

【1】Fires can burn down trees. They can hurt people. They are very dangerous. We must be careful with fires. What must we do when there is a fire? We must go outside quickly and never take the lift. We must call 119. We have a fire drill at school every year.

【2】A fire drill is not a real (真实的）fire. It helps us prepare (准备) for a real fire. First, the fire alarm sounds. Next we all line up in the classroom. Then we leave the classroom quickly. We mustn't run. We must leave all our things in the classroom.

【3】Then we walk down the stairs and go outside. We line up in the playground. Our teacher checks on us. We wait for the firemen to come.